Ok, dont' get your hopes up too high just yet. I've found something called a Flexmode display driver on a set-top box called
Vario. Basically, it modifies the standard intel 845 chipset display driver to allow 848x480. I know that's not quite 800x480 but it's defintly a non-standard resolution.
This document here (from downloads) shows detailed info on the bits required to switch the display timing on the chip. My guess is that it's all a matter of the driver matching up the display's specs.
I tried to install the software on my commell motherboard, which has the intel chipset but kept getting an "unsupporte feature" error or something like that. I'll be having another attempt at some point, but thought I will share this with others. I'm hoping that someone more clued up than me on vga drivers etc. may be able to use this info.